The predominant type of domestic, private sewage disposal system is the septic tank and drainage field. Waste from the house flows into a water tight septic tank. There it remains for approximately 24 hours while bacteria reduce many of the solids to a liquid state. At the end of this interval, the liquid flows out of the tank into a leaching field through a network of porous pipes. These porous pipes are placed fairly close to the ground surface, which allows air to get to the liquids and evaporate them before they have a chance to drain off into the ground water.
It has been found that although most of the solids contained in the sewage slurry are biodegradable, a substantial portion of these solids are not decomposed within the approximately 24 hour period that a given volume of the slurry remains in the septic tank. Thus the longer term biodegradable solid components of the slurry which flow out of the septic tank into the network of porous pipes, will begin to accumulate therein if their rate of production in the sewage system is greater than their rate of disintegration.
The early prior art in septic tank drainage field construction employed one foot lengths of vitrified clay tile pipe which were laid end-to-end with one inch separations therebetween, in a drainage ditch lined with crushed rock. Asbestos shingles or tar paper would then be placed over the one inch wide gap between adjacent clay tiles to enable the sewage slurry to seep therethrough without admitting the crushed rock lining the drainage trench to enter into the tile interior. This early prior art system worked well, allowing larger non-degraded solids such as non-degraded tissue paper to pass out of the drainage tiles while excluding the admission of the crushed rocks lining the drainage trench into the clay tile interior.
More recently, however, the use of vitrified clay tile in septic tank drainage field construction, has been replaced by the use of less expensive plastic pipe having circular holes axially displaced along the bottom thereof to accomplish the fluid distribution function. It has been found, however, that the larger solid components of the sewage slurry will not pass through the circular holes in the prior art plastic drainage pipe. Simply enlarging the circular holes will lead to an alternate problem of admitting the crushed rock which lines a drainage trench, into the interior of the plastic pipe thereby further clogging it.